Production lines and methods of the above indicated type are conventionally used in the large-scale series production of cast parts. The Applicant, for example, thus operates a production line with which motor units are cast in large numbers in an automated sequence in the described manner. In the known production line a number of core shooting machines are linearly linked together for this purpose. The number of core shooting machines required for this corresponds to the set of tools available in each case for a complete core package of a specific type of motor unit.
The shot and completely hardened cores are removed via removal palettes and assembled one after the other on an assembly line set up parallel to the core shooting machines to form a core package. To ensure the economic efficiency of a production line of this type cycle times of less than 60 seconds have to be adhered to with corresponding expenditure on automation.
A moulding material mixed from a known organic binder and a likewise conventional moulding sand is used in the known production line as the moulding material for producing the cores. This moulding material is compacted in what is referred to as the “cold box method” in which the organic binder is hardened by gassing with a reactive gas. The finished casting cores are assembled to form the casting moulds, temporarily stored in a storage device for gas evolution and subsequently mechanically tensioned together in the casting unit and cast.
After casting of the molten metal the respective casting mould is brought into a solidifying position, starting from which it passes cast part specifically in the pretensioned state through a cooling section for a period longer than 15 minutes. After solidification the casting moulds are loaded on palettes and moved into a heat treatment furnace. In this furnace the cast parts (motor units) are thermally desanded and solution treated in a process lasting several hours.
During thermal desandling the organic binder of the casting moulds is broken down at temperatures in the cast part that are just under the solidus temperature of the alloy used, so the sand casting mould disintegrates in rough fragments. As a result of further heating, mechanical conveying device and screening as well as the use of expensive sand coolers and bunkers the core shop is then supplied with fine-grain recycling sand again. Large quantities of sand and long conveying distances are required owing to the protracted thermal process.
An automated casting plant is also known from DE 40 16 112 C2, wherein a plurality of functional units are provided which are connected to a production line by intermediate conveyors.
Known production lines of the above described type allow motor units to be produced cost effectively in large numbers but are confronted by operating drawbacks which are particularly noticeable if smaller numbers are to be produced or the models of the parts to be cast frequently change. Thus greater technical expenditure caused by a number of machines and tools is required for core production. During a tool change, which becomes necessary following a change of model, the large number of complex machine units and the compulsion to run cycle times of less than 60 seconds entails long set-up times and complex assembly work which, in turn, cause availability losses. These losses induce low flexibility in the known production line as high set-up costs and, in the case of new products, additional investment costs stand in the way of quick adaptation to changed operating conditions or model types. All devices must be configured for it to be possible to achieve a short cycle time for each product.
The use of cores bound with organic binders also entails the problem that the tools used for producing the cores have to be cleaned outside of the core shop at regular intervals. Expensive exhaust air systems are also required to collect and purify the gases occurring during hardening of the cores in the “cold box method” and during thermal combustion.
These gases also lead to corresponding stresses on personnel. Casting defects can occur during the casting process as a result of gassing cold box cores.
A further drawback of the known production lines that entails high operating costs resides in the necessity of using a furnace with long treatment times for heat treatment and fettling, the furnace providing such high temperatures that the binder of the casting moulds is broken down and solution treatment is carried out at the same time. The flexibility with respect to a variation in the heat treatment parameters is severely limited by coupling to the thermal desanding.
The purely thermal desanding proves to be a problem in the case of sand adhesions (penetration, organic condensates), in particular in the inner channels of a motor unit.
High expenditure for the sand circulation owing to high sand temperatures, large quantities of sand, the necessity of cooling the sand to a defined temperature and the very high space requirement for the furnace additionally contribute to the fact that the known production lines may only be operated economically if the same motor units are produced in high numbers over a long production cycle. This economic efficiency consideration is opposed by the fact that the development times in the new construction of cast parts, in particular in the field of motor development, are becoming increasingly shorter and the changes in model accordingly becoming increasingly more frequent.